Not to me. One of my bad decisions, thinking Fastmail would stand by its word. Fastmail offered something clearly worth more than $15 at $15. So I bought it hoping for an email address good for life. Fastmail changed their mind, so now I am screwed: I made a bad choice. Would receiving $15 back help, NO. Would receiving a discount on Fastmail help, NO. At least most of the people I have recommended Fastmail to have already moved away from Fastmail. My significant other will be a significant problem however, but at least she has a guest account instead making it somewhat easier.

2) Did it become worth more with Fastmail's enhancements?

Not to me it didn't. I just wanted somewhere to receive 0-5 text emails a day, occasionally needing to see the html version.

Aside: It hasn't been possible to see the html version in webmail since the new system(realizing text has to be the default view given the above). Although why anyone would use webmail has always been a little bit of a mystery.

3) Will transferring to a new email address be easy?

Not particularly, mostly since members have been treated worse than guests - I've long wished I'd kept my guest account instead of upgrading(since many years ago downgrading) to member status. With such small storage in the member account, there is not much trash to look at to see who uses the Fastmail account.

Hey, but at least I'll gain an address to give out to businesses likely to send a lot of email I don't want.

4) Are threats of a class action lawsuit silly?

I personally wouldn't consider such a thing, but here is a situation where Fastmail has all the power and the user has none. That sounds like a potential lawsuit situation. And it's not a lawsuit over $15: clearly Fastmail knowingly mispriced the member category, just read this thread. If it was really worth thousands of dollars as some are intimating, then it is a lawsuit over thousands of dollars, Fastmail's mispricing has little to do with what the product is/was worth. Lawsuit would not prevail though.